1. Field of the Invention
The present invention relates to a fluid filled type vibration damping device which is adapted to provide vibration damping action based on flow effects, such as the resonance effect, of fluid through an orifice passage interconnecting a pressure receiving chamber and an equilibrium chamber which are filled with a non-compressible fluid. In particular, the present invention is concerned with a fluid filled type vibration damping device which is furnished with a mechanism for absorbing pressure fluctuations in the pressure receiving chamber.
2. Description of the Related Art
A fluid filled type vibration damping device which affords vibration damping action based on flow effects of a non-compressible fluid which fills its interior is one type of vibration damping device known in the art as vibration-damped coupling or vibration-damped support or the like for installation between components which make up a vibration transmission system. This fluid filled type vibration damping device has a first mounting member and a second mounting member which are positioned spaced apart from one another and which are elastically linked by a main rubber elastic body; and a fluid chamber filled with a non-compressible fluid. The fluid chamber is divided by a partition member which is fixedly supported on the second mounting member thereby respectively forming to either side of the partition member a pressure-receiving chamber a portion of whose wall is defined by the main rubber elastic body, and an equilibrium chamber a portion of whose wall is defined by a flexible film, with the pressure receiving chamber and the equilibrium chamber communicating with one another through an orifice passage. With this structure, vibration damping action will be produced on the basis of flow effects such as the resonance effect of fluid flowing through the orifice passage when relative pressure fluctuations between the pressure receiving chamber and the equilibrium chamber occur in association with input of vibration. Implementation of such fluid filled type vibration damping devices in automotive engine mounts, body mounts, diff mounts, or other suspension mounts for example, is a topic of ongoing research.
Vibration damping action which is produced through flow action of a fluid through an orifice passage will be limited to a relatively narrow frequency band to which the orifice passage has been pre-tuned. A resultant problem is difficulty in dealing with the sophisticated vibration damping characteristics which are required.
Accordingly, there has been proposed for example a pressure fluctuation absorbing mechanism which has as an object to suppress the sharp rise in pressure fluctuation occurring during input of vibration which can be a problem in frequency ranges higher than the tuning frequency of the orifice passage, and to thereby stabilize vibration damping action. As taught in U.S. Pat. No. 4,697,793, this pressure fluctuation absorbing mechanism has a structure in which a movable member composed of a rubber film is installed in the partition member which divides the pressure receiving chamber from the equilibrium chamber so that pressure of the pressure receiving chamber will act on a first face and pressure of the equilibrium chamber will act on the other face. The movable member will experience deformation in association with input of vibration in a high frequency band to thereby absorb pressure fluctuation, resulting in avoiding development of high dynamic spring.
In fluid filled type vibration damping devices of the type described above, there are instances in which input of large, jarring vibration load across the first mounting member and second mounting member may cause the vibration damping device to emit noise. Specifically, where the fluid filled type vibration damping device is employed as an automotive engine mount for example, noise and shock of sufficient intensity to be perceptible to passengers in the cabin may be produced when the car drives over grooved pavement, speed bumps, or the like.
Formation of air bubbles, known as cavitation, is a possible cause of such noise and vibration. Such bubbles are produced since the flow of fluid through the orifice passage between the pressure receiving chamber and the equilibrium chamber cannot keep pace with input of a large, jarring vibration load, resulting in excessive negative pressure within the pressure receiving chamber. It is thought that these bubbles then burst, producing water hammer pressure which is propagated through the first mounting member and the second mounting member and transmitted to the car body etc., resulting in noise and vibration which poses a problem as noted above.
U.S. Pat. No. 4,697,793 discloses a fluid filled type vibration damping device as one measure for dealing with this problem. In this fluid filled type vibration damping device, an incision is made in the movable member which is supported on the partition member, so that in the event of an excessive relative pressure differential between the pressure receiving chamber and the equilibrium chamber the movable member will undergo deformation on the basis of the pressure differential, causing the slit to open up so that the pressure receiving chamber and the equilibrium chamber now communicate with one another through the opening. The excessive negative pressure in the pressure receiving chamber will be dispelled thereby so as to suppress the occurrence of cavitation.
However, with the provision of an incision to the movable member, there is risk that, even in instances where excessive positive pressure at a level not requiring the incision to open has been produced in the pressure receiving chamber, the movable member will undergo appreciable elastic deformation causing the incision to open up. As a result, shunting between the pressure receiving chamber and the equilibrium chamber may occur even when the input vibration lies in the tuning frequency band which the orifice passage is intended to damp, resulting in failure to effectively create a relative pressure fluctuation differential between the pressure receiving chamber and the equilibrium chamber, and a consequent insufficient flow of fluid through the orifice passage with an attendant risk that sufficient vibration damping action (high attenuating action) based on flow effects such as the resonance effect of fluid through the orifice passage will be achieved with difficulty.
In view of this problem, in U.S. Pat. No. 7,306,210, the Applicant proposed a structure in which a shunt passage is formed in the partition member to connect the pressure receiving chamber and the equilibrium chamber; a valve body is provided for switching the shunt passage between a communicating state and an obstructed state; and a metal spring is provided for maintaining the obstructed state with the valve body at a prescribed level of initial elastic deformation. With this arrangement, with the pressure receiving chamber under conditions of pressure of ordinary magnitude, the shunt passage will be maintained in the obstructed state through the elastic deforming action of the metal spring so as to ensure a sufficient level of fluid flow though the orifice passage. On the other hand, with the pressure receiving chamber in a condition of excessive negative pressure that may pose a problem, the shunt passage will assume the communicating state in opposition to the elastic deforming action of the metal spring, thus preventing a state of excessive negative pressure from arising in the pressure receiving chamber.
However, providing the partition member with the pressure fluctuation absorbing mechanism described above in addition to the valve results in an overall complicated structure including the partition member, and poses the additional risk of difficulty in assuring sufficient space in the partition member to accommodate the pressure fluctuation absorbing mechanism of the pressure receiving chamber and the shunting mechanism between the pressure receiving chamber and the equilibrium chamber. Hence there was also a risk of difficulty in achieving the objective vibration damping action and noise reducing action on a consistent basis.